The present invention relates in general to mixer circuits, and more particularly to doubly balanced mixer circuits characterized by a significant reduction in distortion.
The ever increasing number of R.F. signals present in our environment presents problems for modern communication systems to achieve a high dynamic range. Mixers are generally a dominant contributor to internally generated communication system distortion products that reduce system dynamic range. Certain of these distortions products may be avoided by choosing system frequencies that do not cause interference products to occur or by using frequency selective networks or filters at each of the mixer's input and output ports. Some of these distortion products become impossible to avoid and also to eliminate by use of frequency selective networks.
One of such distortion products that occurs when multiple RF signals are present may be expressed in the form (F.sub.LO .+-.F.sub.RF1 .+-.2F.sub.RF2) and (F.sub.LO .+-.2F.sub.RF1 .+-.F.sub.RF2), commonly referred to as 3rd Order intermodulation Distortion (IMD). This set of distortion products normally cannot be removed by the use of frequency selective networks or filters because the desired RF input frequencies (F.sub.RF1) and (F.sub.RF2) may be too closely spaced in frequency to allow the use of a realizeable filter network. Also, internally generated mixer IMD increases dramatically when frequency selective networks or filters are connected directly to the ports of a conventional mixer constructed utilizing prior art techniques. The effects of IMD on mixer operation is described by Peter Will in "Reactive Loads--The Big Mixer Menace," published in the April 1971 of Microwaves, Vol. 10, No. 4, p.p. 38-42, and "Uncover Mixer Intermod with Swept Meausrements," Vol. 17, No. 11, p.p. 84-91, November 1978.
Conventional doubly balanced mixer circuits rely upon the symmetry or balance of both diodes and transformers for cancellation or supression of even order (2F.sub.RF1, 2F.sub.RF2) frequency terms. These circuits may be considered to have the local oscillator current from a local oscillator input signal flowing in series through each one of the diodes in the alternately conducting pair of diodes, while the RF current from an RF input signal flows through the same pair of diodes in parallel. It is possible to have different values of current flow in each conducting diode causing an unbalanced condition.
Cancellation effects of IMD can occur when one diode in each of the alternately conducting pairs of diodes are biased at different levels as described by J. H. Lepoff and A. M. Cowley, in "Improved Intermodulation Rejection in Mixers" published in IEEE Transactions on Microwave Theory and Techniques, MTT vol. 54, pp. 616-622, December, 1966. Conditions similar to those responsible for IMD cancellation can also cause serious IMD increases. Since a doubly balanced mixer is really a "bi-phase" modulator, wherein the RF port is commutated to the IF port with a 180.degree. phase reversal ocurring during each half of the LO cycle, the mixer's RF input impedance is theoretically a direct function of its IF port termination impedance. When an RF signal source is connected to the RF port of a mixer that has a completely reflective IF port termination, the RF input voltage or RF input current acting upon the diodes may be up to twice the matching impedance RF voltage or twice the matched impedance RF current available to the mixer under nonreflective termination conditions. IMD products generated by each of the diodes higher order 4th and 6th coefficients of a series expansion around an LO bias point are summed at the mixer's IF port. It has been determined that large increases in mixer generated IMD is a result of an impedance mismatch or reflective IF port termination. Examples of such termination sensitive mixers are Anzac Model MD-123, MD-525 mixers and the similar mixer described in U.S. Pat. No. 4,063,176.
In an effort to reduce mixer generated IMD, designers have evolved further classes of doubly balanced mixers wherein each diode is replaced by the various circuit element combinations. These efforts have only met with partial success and normally require an increase in local oscillator input power to realize any improvements in mixer generated IMD.
Accordingly, it is an object of the present invention to provide a novel broadband frequency mixer adapted to efficiently operate despite non-ideal reflective source and load terminations.
Another object of the present invention is a reduction of mixer generated IMD when the mixer is operating with non-ideal reflective source and load terminations.
A further object of this invention is to minimize any changes in mixer generated IMD under any combination of source and load impedance conditions.
A further object of this invention is to provide an improved doubly balanced mixer in which all of the mixer's signal ports are mutually interchangeable.
Other objects, advantages and capabilities of the present invention will become more apparent as the description proceeds taken in conjunction with the accompanying drawings, in which like parts have similar reference numbers.